The present invention relates generally to the field of fiber optics, and particularly to advantageous aspects of systems and methods for fabricating a varying waveguide optical fiber device.
Optical fibers are thin strands of glass material designed to carry optical signals long distances at high bandwidth. Optical fibers are typically cylindrically symmetric structures, including an inner core region and one or more distinct cladding regions outside the core region. As used herein, the terms xe2x80x9ccladdingxe2x80x9d and xe2x80x9ccladding regionxe2x80x9d refer generally to regions of an optical fiber outside of the core region. Depending upon the particular fiber design, a cladding region may not necessarily have an annular cross section and may not necessarily be radially symmetric. One or more of the core and cladding regions are typically doped with various chemicals at various concentrations to create a desired refractive index profile.
One device that is useful for a number of applications is an optical fiber having a refractive index profile that varies along its length. Various approaches have been developed to fabricate such a device. In one approach, the refractive index profile of an optical fiber is modified by altering the fiber diameter to produce a physical tapering of the fiber. In a second approach, dopants in an optical fiber are diffused to modify the fiber""s refractive index profile. In a third approach, an optical fiber""s refractive index profile is modified by irradiating the fiber with high intensity UV radiation.
However, there are a number of known disadvantages of the prior art. Depending upon the particular technique used, it may be difficult to obtain a desired change in refractive index in a short length of fiber. Also, it may be difficult to precisely control the modulation of the refractive index profile in a length of fiber.
Aspects of the present invention provide systems and methods for fabricating a varying-waveguide optical fiber. In one method according to an aspect of the invention, a preform is fabricated having a core and at least one cladding region. The cladding region has a higher viscosity and the core region has a lower viscosity. The relative viscosities of the cladding region and core are chosen such that, when tension is applied to an optical fiber drawn from the preform, the applied tension is primarily borne by the cladding region, thereby causing a viscoelastic strain to be frozen into the cladding region, the frozen-in viscoelastic strain decreasing the cladding region refractive index and producing a net change of the cladding region refractive index relative to the core refractive index. The method further includes drawing the preform into an optical fiber under an applied tension, such that a viscoelastic strain is frozen into the cladding region, the frozen-in viscoelastic strain decreasing the cladding region refractive index. The cladding region refractive index is changed in a section of the optical fiber by heating the section so as to relax the viscoelastic strain frozen into the cladding region in the section of fiber, thereby increasing the cladding region refractive index in the section of fiber. This heating may also cause a relaxation of elastic stresses frozen into the core region, thereby inducing small changes in the core refractive index.
Additional features and advantages of the present invention will become apparent by reference to the following detailed description and accompanying drawings.